CT Appearance of Cysts and Blood
Cysts appear as homogeneous, low-density lesions measuring <20 Hounsfield units (HU) on unenhanced CT, while blood or hemorrhagic content increases attenuation to 20-70 HU or higher, making them appear hyperdense and potentially mimicking solid masses. 1
Cyst Appearance on CT
Simple Cysts
- Homogeneous masses measuring <20 HU on unenhanced CT are definitively benign cysts and require no further imaging characterization 1
- On contrast-enhanced CT, homogeneous masses measuring 10-20 HU are benign cysts 1
- Recent evidence shows that homogeneous masses measuring 21-30 HU on portal venous phase contrast-enhanced CT may also be benign cysts 1
- Simple cysts appear as thin-walled structures with homogeneous low-density interior 1
- The cyst wall is typically not visible, and fluid is isodense with cerebrospinal fluid 1
Complex/Hemorrhagic Cysts
- Any mass with density 20-70 HU on unenhanced CT is indeterminate and warrants further evaluation 1
- Hemorrhagic cysts show heterogeneous appearance with increased attenuation due to blood products 2
- High protein content can also cause increased attenuation (up to 35 HU) in benign cysts, mimicking solid lesions 3
- Calcification of the cyst lining may develop after hemorrhage and is visible on CT 2, 4
Blood Appearance on CT
Acute Hemorrhage
- Fresh blood appears hyperdense (high attenuation) on CT, typically measuring 50-70 HU or higher 2
- Hemorrhagic content causes cysts to appear in the 20-70 HU range, making them indeterminate 1
- CT can detect extravasation of cyst contents in rare cases of cyst wall rupture 2
Chronic Blood Products
- Blood breakdown products may show variable attenuation depending on age 5
- Fluid-fluid levels may be visible, representing blood-filled lakes between septa 2
Critical Diagnostic Pitfalls
Pseudoenhancement
- Small renal masses (≤1.5 cm) are challenging to evaluate due to pseudoenhancement phenomenon 1
- Partial volume averaging limits assessment of true enhancement in small masses 1, 6
- Using 5-mm thin sections instead of 10-mm slices reduces partial volume artifact and improves differentiation between cysts and solid masses 6
Mimicking Solid Masses
- Hemorrhagic or proteinaceous cysts frequently appear as solid masses on CT due to increased attenuation 7, 3
- Bronchogenic cysts with density >20 HU are often mistaken for solid tumors 7
- Eight of 11 bronchogenic cysts in one series appeared solid based on HU measurements 7
When CT is Insufficient
MRI Superiority
- MRI is highly specific for diagnosing hemorrhagic cysts, showing heterogeneous intense signal on both T1- and T2-weighted sequences 2
- MRI definitively proves cystic nature of indeterminate non-water attenuation masses on CT 1
- MRI shows characteristic fluid-fluid levels and hyperintense internal septations on T1-weighted images corresponding to hemorrhagic content 2
Contrast-Enhanced Ultrasound
- CEUS shows lack of enhancement of intracystic structures, highly suggestive of clotting rather than solid tissue 2
- CEUS successfully classified 95.7% of previously indeterminate lesions in one study 1
Practical Algorithm
For masses measuring:
- <20 HU unenhanced: Benign cyst, no further workup 1
- 20-70 HU unenhanced: Indeterminate, requires multiphase contrast CT or MRI 1
- >70 HU unenhanced: Benign (likely hemorrhagic/proteinaceous cyst or contains macroscopic fat) 1
- 10-30 HU on portal venous phase contrast CT (if homogeneous): Likely benign cyst 1
If density remains indeterminate after contrast CT, proceed to MRI for definitive characterization of hemorrhagic versus solid content 1, 2